1. Field of the Invention
The present invention pertains to a method of and an apparatus for correcting a contour of an image portion in an integrated image including plural image portions.
2. Description of the Background Art
In multi-color printing, a dropout (or a non-filled area) is sometimes observed on the border between different image portions or regions (hereinafter referred to as image parts) due to registration mismatch or misalignment among plural color plates. FIGS. 1A through 1C are schematic views illustrating examples of registration mismatches of a printed image including two image parts.
FIG. 1A shows an image registered accurately. Distributions of yellow density Dy and magenta density Dm over two image parts R10 and R20 are shown in the lower part of FIG. 1A. The region R10 is a tint region, i.e. a region of a uniform dot % (or a uniform halftone-dot area rate), having a yellow component of 80 dot % and a magenta component of 20 dot %, and the region R20 is a tint region having a yellow component of 20 dot % and a magenta component of 80 dot %.
FIG. 1B shows an example of an image with a registration mismatch. A yellow plate is shifted leftwards and a magenta plate rightwards from their accurate positions shown in FIG. 1A. Here a new region Ra is formed between the regions R10 and R20, whose yellow and magenta components are both 20 dot %. Hence, the region Ra is lighter in color than the other regions R10 and R20 and is consequently more white. Such a region as Ra is not included in the correct printed image and is clearly distinguished from the other regions because of its whitish shade; thus it deteriorates the appearance of the printed matter.
In the instant specification, a dropout or a dropout region designates a region, e.g., region Ra of FIG. 1B, that is formed due to registration mismatch among plural color plates and which has a lighter color than the surrounding regions as well as a region in which no inks are applied.
FIG. 1C shows another example of an image due suffering from registration mismatch. The yellow plate is shifted rightwards and the magenta plate leftwards from the accurate positions shown in FIG. 1A. Here a new region Rb is formed between the regions R10 and R20, whose yellow and magenta components are both 80 dot %. Hence the region Ra is darker than the other regions R10 and R20 and is blackish consequently. The correct printed image does not include a region such as Rb; the region Rb is not distinguishable from the other regions but looks like part of a boundary line because of its blackish shade.
When a printed image is formed while printing plates are mis-registered as shown in FIG. 1B, the dropout region Ra deteriorates the printed image. Plates for forming a printed image as shown in FIG. 1C are currently preferred. Hereinafter in the specification, "processing for correcting image contours" denotes processing for correcting contours of image parts to form such preferred printing plates. When contours of image parts are corrected adequately, occurrence of a dropout region will be prevented even in the case of registration mismatch as shown in FIG. 1B.
Such processing for correcting image contours is not easily performed in consideration of respective dot percents of adjacent image parts and thus requires skilled operators.
FIG. 2 is a schematic view showing the difficulty in processing to correct image contours. The printed image of FIG. 2 includes three regions R30, R40, and R50. The regions R30, R40, and R50 have magenta components of 50 dot %, 80 dot %, and 10 dot %, respectively. In conventional processing for correcting image contours, an operator determines the shape of a region to be corrected, i.e., a shaded region in FIG. 2, and then specifies a dot % of the region. In FIG. 2, for example, 80 dot % is specified for a range surrounded by the regions R30 and R40 and 50 dot % for a region surrounded by the regions R30 and R50 for preventing an occurrence of a dropout due to registration mismatch.
In the above conventional method, only skilled operators can accurately determine the dimension of a region to be corrected and adequately specify a dot % of the various regions.